The arc tube of crystalline alumina ceramic in a high pressure sodium vapor lamp may be heated in operation to an average temperature of 1000.degree. C. or higher, and the material has a coefficient of linear expansion of 8.times.10.sup.-6 per .degree.C. The arc tube is so hot that the only practical way of supporting it is by the inleads extending from the ends. In a typical 400 watt lamp, the arc tube is about 11 centimeters long and will expand about 1 millimeter. The metal support frame and the vitreous outer envelope will not expand nearly as much, and moreover the expansion of the various parts does not occur at the same time when the lamp is warming or cooling. Accordingly it is necessary to accommodate differential expansion in order to avoid undue stress on the inleads and seals which may lead to fatigue and premature lamp failure through cracking of the seals or rupture of the welds to the inleads. This need is particularly great with ceramic plug type seals such as used in the lamp described in U.S. Pat. No. 4,065,691 - McVey, Ceramic Lamp Having Electrodes Supported By Crimped Tubular Inlead.
In McVey U.S. Pat. No. 3,882,346 - Ceramic Arc Tube Mounting Structure, a differential expansion compensating mounting is described and claimed which has found substantial use with lamps utilizing ceramic plug seals. While that structure is quite satisfactory in service, a mounting which is cheaper and more easily manufactured is desired, and the object of the invention is to provide such a mounting.